Optical systems used in laser printers may be characterized as having three sub-systems or assemblies, namely, a laser diode/pre-scan optical assembly, a scanning assembly, and a post-scan assembly. Typically, the laser diode/pre-scan optical assembly includes a laser diode emitting a diverging laser beam, a collimator lens for collimating the beam emitted by the laser diode, and a pre-scan lens to focus the beam to a waist near the scanning device so that the post-scan assembly images the beam to a waist at a corresponding photoconductive (PC) drum surface.
The scanning assembly generally includes a scanning device such as a motor driven, rotatable polygon mirror having a plurality of peripheral mirror surfaces or facets that rotate during operation of the printer. The mirror surfaces reflect the collimated and focused beam received from the laser diode/pre-scan optical assembly. The direction of rotation of the polygon mirror determines the scan direction of the beam passing along a scanned object, such as a PC drum in a laser printer.
U.S. Pat. No. 4,578,688 to Okuno discloses a printer having two laser diodes emitting two laser beams reflected off of a common polygon mirror. The '688 patent teaches reducing registration errors by having the two laser beams contact the polygon mirror such that “the directions of inclination of the beams La and Lb relative to the imaginary plane perpendicular to the rotary shaft 10 of the polygon mirror 3 are opposite to each other,” see column 4, lines 27-30. Hence, “the directions in which the scanning lines Aa and Ab are curved are the same,” see column 4, lines 30 and 31.
There is a need for a laser scanning unit comprising at least two laser diodes for emitting a plurality of laser beams wherein the corresponding post-scan optical assemblies are configured so that the space on either side of the scanning assembly is efficiently used so as to facilitate minimization of the space requirements of the laser scanning unit. There also is a need for an optical system for a laser scanning unit in which the optical components may be readily aligned and, in particular, a system and a method of alignment in which post-scan optics may be used to readily adjust the process position and bow of scan lines.